Psychoactive substances change the consciousness and mood of their user. The people using them may have pleasant feelings, such as euphoria or they may be more alert. For this reason, many psychoactive substances are abused: They are used outside the aims of the treatment. This may lead to the user developing a psychological and physical dependence ("addiction"). It will get more difficult to interrupt the cycle of abuse.

In part because of this potential for abuse and dependency, the ethics of drug use are the subject of a continuing philosophical debate. Many governments worldwide have placed restrictions on drug production and sales in an attempt to decrease drug abuse. Ethical concerns have also been raised about over-use of these drugs clinically, and about their marketing by manufacturers.